Effects of radiation on fine vasculature may determine the response of dependent tissues late after a course of radiotherapy. Additional stress to irradiated tissues may cause additional injury to fine vasculature causing degeneration leading to acute necrosis. The primary objective of this study is to determine the capability of endothelial cells to repair radiation damage. Removal of layers of the cornea of the eye will cause rapid proliferation of capillary endothelial cells at the limbus. Irradiation of the cornea with sufficiently high doses will prevent endothelial proliferation. The dose required to prevent neovascularization of the cornea in 50% of eyes (NVD50) of dogs is approximately l000 rads. The proposed experiments are designed to determine endothelial cell capability for repair by utilizing split dose techniques. The increase in total dose required to produce a given effect when given as two doses compared to a single dose will indicate this capability. Irradiation done prior to surgery provides data on undisturbed, slowly proliferating endothelial cells. Irradiation 24 hours following surgery provides data for rapidly proliferating endothelial cells. In both intances, the repair capability is equal to 350 rads (approximately). This model system will also be useful for studying comparative effects of negative pi mesons in the region of peak energy distribution. Presumably tissues would have little capability for repair of damage produced in that region.